Temporary Pacing After Cardiac Surgery

Temporary pacemaker leads are placed on the epicardial surface of the heart during cardiac surgery to assist in maintenance of cardiac rate and rhythm postoperatively. In addition to providing a reliable cardiac stimulus in the event of bradyarrhythmias or asystole, pacing wires can be used to optimize cardiac output and assist in the diagnosis and suppression of arrhythmias. Although placement of epicardial pacing leads was once considered essential, pacing is required in only 25% of valve patients and 10% in those undergoing coronary bypass procedures.1,2 This therapy is not without risk; in addition to surgical considerations of lead placement, pacing can potentially reduce cardiac output or precipitate life-threatening arrhythmias, warranting a thorough knowledge of this therapy to promote optimal patient outcomes.The essential components of any pacing system include a generator to initiate an electrical stimulus, which is then transmitted via an insulated lead, to electrodes that are in direct contact with the heart. Unlike permanent pacemaker systems that are completely internal and require sophisticated programmers to modify impulse delivery, temporary pacing generators are larger, external devices used by a wide variety of providers to readily adjust device functions at the bedside. Rather than a transvenous placement approach as used in the cardiac catheterization laboratory, cardiac surgical electrodes are inserted on the outside of the heart when the chest is open. During surgery on the heart, the pericardium is routinely opened for access to surgical targets, exposing the epicardial surface for electrode placement at the completion of the procedure. These small wires are secured to a sewing needle that is pulled shallowly through a small portion of myocardial tissue and then pushed through the chest wall and out through the skin. The sharp end of the needle is then broken off and discarded, leaving a short metal tip for attachment to the generator.3 With a temporary pacing system, a connecting cable is routinely used to carry the impulse between the leads and generator, functioning like an extension cord to allow the generator to be remote from the chest.Two electrodes are required to generate an electrical signal: a negative pacing electrode (anode) must remain in direct contact with the atrium and/or ventricle to deliver current to the myocardium, while a positive or grounding electrode (cathode) returns the signal to the generator to complete the electrical circuit. Numerous electrode configurations can be used to create either a bipolar or unipolar temporary pacing system. A single, bipolar lead can be used that contains both the cathode and anode located several millimeters apart; with both electrodes in direct contact with the heart’s surface, either can potentially serve as the anode and generate the signal (Figure 1). The polarity of a bipolar lead is, therefore, determined by its connection to the generator: the lead that is connected to the positive pole of the generator/connecting cable will be the cathode, while the lead connecting to the negative pole serves as the anode and delivers the pacing stimulus. A bipolar pacing system can also be created by placing 2 separate unipolar electrodes on the epicardium in close proximity, with polarity again determined by lead connections to the generator or connecting cable. Regardless of whether 1 or 2 leads are used, a bipolar pacing circuit offers the advantage of redundancy; should 1 electrode fail, changing the polarity of cable connections activates the opposite electrode to begin pacing. Potential disadvantages of bipolar pacing include sometimes small, difficult-to-see pacing artifacts, particularly with atrial leads placed close together. In addition, every electrode that passes through the heart has the potential to bleed.In contrast, a unipolar pacing system includes only 1 electrode placed on the epicardium. The cathode is inserted remotely from the heart by sewing an additional lead onto the outer skin surface (Figure 2). Although technically this lead could be placed anywhere on the skin, it is typically secured to the external chest wall in close proximity with the other unipolar lead, thus limiting the current path and potential for electrical interference.3 A unipolar system generates a large pacing artifact and also creates fewer potential sites of bleeding around the heart.Using these configurations, separate pacing circuits can be created to provide atrial and/or ventricular pacing by securing electrodes to the right atria or right ventricle, respectively. By convention, atrial wires exit the patient’s right chest while ventricular wires are located to the left of midline. Atrial versus ventricular pacing settings are distinguished by ensuring their proper connection to the A (atrial) or V (ventricular) receptacles at the top of the pacing generator. Adjustments to pacing modes and generator settings can then be made to optimize cardiac function.The optimal pacing mode depends on the type of epicardial wires used and patient need. Nomenclature for temporary pacing modes uses a 3-letter code to describe the chamber(s) paced, chamber(s) sensed, and mode of operation, whereby A = atrial, V = ventricular, D = dual, O = none, and I = inhibited.4 Pacing options include the synchronous modes of DDD, AAI, and VVI, which allows the device to sense intrinsic cardiac activity and inhibit competitive pacing, ideally responding only “on demand” should pauses occur. Asynchronous dual and ventricular pacing modes (ie, DOO and VOO) are not advised, as the lack of sensing capability creates a risk for arrhythmia from the “R-on-T” phenomenon, should a pacing stimulus occur during the vulnerable period of the cardiac cycle.5 These modes are reserved for use in the operating room if electrical interference from cautery devices occurs, as well as during asystole or other emergency situations. An advantage of having both atrial and ventricular pacing wires is the ability to use the DDD pacing mode, which allows the device to operate in its most physiological manner and be either inhibited or triggered in response to the underlying rhythm. The electrocardiogram (ECG) will reveal 1 of 4 patterns in DDD mode: an intrinsic rhythm (A and V sensing), 100% pacing of atria and ventricles, atrial pacing followed by a native QRS, or sinus P waves followed by a paced QRS. The final 2 examples demonstrate a triggered pacemaker response; once atrial pacing or sensing has occurred, this triggers a timer for the AV interval. If an intrinsic QRS arrives within that interval, ventricular pacing is inhibited. If no intrinsic QRS occurs within this AV delay period, then ventricular pacing is triggered.The appropriate setting for heart rate (HR) generally ranges from 80 to 110 beats per minute (bpm) postoperatively and is adjusted to provide an optimal cardiac output, whereby cardiac output = HR × stroke volume. In general, increasing the pacing rate within this range should improve cardiac output, as long as atrial leads are in place and A-V synchrony is maintained. In the absence of atrial pacing leads, VVI pacing will maintain the HR but without an “atrial kick.” For patients with sinus bradycardia, this loss of atrial contribution to cardiac output may negate any benefit of increased HR during ventricular pacing. In addition, rate adjustments may prove beneficial in limiting postoperative arrhythmias. The Atrial Fibrillation Suppression Trial–II (AFISTII) investigators demonstrated that AAI pacing at 100 to 110 bpm, in combination with amiodarone, was effective in reducing the incidence of atrial fibrillation to 15.8% versus 37.5% in control patients.6Separate A and V settings are available to adjust the amount of electricity, in milliamperes (mA), used to stimulate the heart chambers. Default settings for this parameter are 10 mA, which should prove adequate for most patients, with upper limits of 20 (A) and 25 (V) mA. The minimal amount of energy required to capture the myocardium and generate a paced beat is defined as the pacing threshold, which is determined from 100% pacing by gradually reducing the output, while continually watching the rhythm for loss of capture. Once established for each chamber, this value is increased 2- to 3-fold to promote continued pacing over time.7 Failure to capture, evidenced by a pacing artifact or “spike” without a corresponding P wave or QRS complex, is a potentially life-threatening situation that may be avoided with adequate outputs and secure connections. Although there is no danger to the heart in using high outputs, high energy use will affect battery life. More importantly, patients who require continuing increases in milliamperes to maintain pacing should be considered at risk for loss of capture and use of alternate pacing systems explored. Troubleshooting for acute loss of capture includes maximizing the milliamperes, ensuring that all connections are secure, or depressing the “EMERGENCY” button on Medtronic devices, which provides maximum outputs in an asynchronous mode, thus eliminating oversensing as a cause of pacing failure. In patients with bipolar pacing systems, switching the terminals of the 2 ventricular pacing leads will change their polarity such that the cathode, when moved to the negative terminal of the pulse generator, becomes the anode, which may be better positioned to pace the heart.This setting determines the ability of the pacemaker to detect or “sense” intrinsic beats, resulting in suppression of the device in synchronous pacing modes. Sensitivity settings determine the amplitude or height required to recognize an intrinsic ECG complex, in millivolts (mV), which signals that pacing is not required. Pacemaker sensing occurs when an impulse is tall enough to cross this sensing threshold; it does not consider the origin of the sensed beat, only its height. Therefore, separate sensitivity settings are essential to distinguish the need for atrial detection of P waves (default 0.5 mV) versus ventricular detection of the QRS complex (default 2 mV). The minimal complex height that is recognized by the device and suppresses pacing is defined as the sensing threshold, which is determined by setting the rate below the patient’s underlying rhythm with 100% sensing (and minimal output as sensing failure will be induced). Gradually increase the sensitivity parameter to make the device less sensitive, while continually watching the rhythm for loss of sensing. Once determined for each chamber, this value should be decreased by approximately half to promote continued sensing.7 Failure to sense, evidenced by a pacing spike in or near a complex when there was no indication for pacing, can be life-threatening should an R-on-T event trigger ventricular fibrillation.5 Maintaining the patient in a synchronous pacing mode (ie, AAI, VVI, or DDD), as well as maintaining appropriate sensing thresholds, is highly recommended to prevent such arrhythmias.3–5,7When operating in a dual-chamber mode, this parameter determines the time delay, in milliseconds, from the onset of an atrial signal (either P wave or atrial spike) to the subsequent ventricular QRS or pacing spike, similar to the P-R interval of the ECG. Because of time limitations imposed by the cardiac cycle, the A-V delay will autoadjust on the basis of the HR. For example, an HR of 60 bpm allows 1 second for each complete cardiac cycle and the A-V delay can be increased up to a maximum of 280 ms, whereas an HR of 90 bpm shortens this cycle and the maximum A-V delay is 240 ms. Prolonging this parameter can be quite useful in fostering intrinsic ventricular activity in patients needing rate stimulation, thus promoting more physiological cardiac contraction versus right ventricular (RV) paced beats, while encouraging return of native conduction. In cardiac surgery patients with impaired ejection fraction, Rubin-stein et al8 demonstrated increases in mean arterial pressure of 4% to 7% and cardiac output of 8% to 14% when AV delay was optimized from default values. They found that a median AV delay of 180 ms was optimal for most of patients, noting that “after cardiac surgery, default values of 120–150 might be too short for diastolic filling.”8 (p If the device indicates “PACEMAKER BLOCK POINT REACHED” and further A-V prolongation is desired, lowering the HR (as tolerated) will allow additional time to further lengthen this parameter.Biventricular pacing is an established therapy for select heart failure patients with intraventricular conduction delay, using permanent devices that stimulate both ventricles simultaneously to generate a more synchronized contraction.9 Results with temporary biventricular pacing after cardiac surgery have been mixed, with some studies demonstrating cardiac output increases of 10% to 20% using biventricular versus RV pacing8,10 whereas others have shown little or no benefit with this therapy.11,12 As multiple factors such as electrode placement and parameter adjustments contribute to these findings, this technique continues to gain favor and is routinely used at the authors’ institution for select patients with impaired cardiac function, which is accomplished using an additional unipolar epicardial electrode inserted on the left ventricle in a similar fashion to the RV lead. Connection of both the RV and left ventricle leads together in the negative output terminal of the generator allows paced impulses to be conducted simultaneously to the ventricles.3Placement of atrial pacing wires at the time of surgery offers several additional adjuncts for the diagnosis and management of postoperative atrial arrhythmias, which occur in 15% to 60% of patients undergoing heart surgery.13 When the rhythm is uncertain, atrial leads can help clarify the diagnosis by enhancing atrial activity that might otherwise be obscured. An atrial ECG can be obtained by connecting the patient to the ECG machine in a standard fashion, with the exception of the V 1 lead. When this lead is instead connected to the metal tip of either atrial wire (removing it from the connecting cable and clipping it to the V 1 lead of the ECG machine), any underlying atrial activity will be enhanced and magnified, appearing as large pacing artifacts in the V 1 view of the atrial ECG that will overshadow the QRS complex.14 Careful analysis of these artifacts can prove invaluable in ascertaining if atrial activity is present and, based on its timing and location, confirm a diagnosis of sinus versus junctional rhythm, atrial flutter, etc, as illustrated in Figure 3.One additional use for atrial leads is the ability to provide rapid overdrive pacing to terminate a supraventricular tachycardia, particularly atrial flutter.4 Using a standard temporary pacing generator in the Rapid Atrial Pacing (RAP) mode, clinicians should first ensure that the atrial leads are connected properly to the atrial terminals to avoid rapid ventricular stimulation. After increasing the atrial output to its maximum of 20 mA, select the RAP parameter on the lower screen of the generator, which enables a menu for the RAP device rate, which is adjusted to 10 to 40 bpm above the intrinsic atrial rate (typically > 300 bpm). While continually depressing the “SELECT” button, pacing impulses will be delivered at this fast rate via the atrial leads. Careful inspection of the monitor while pacing may reveal a change in P wave morphology, indicating capture of the atria at this fast rate. After several seconds of continued overdrive pacing, reduce the RAP rate either abruptly or gradually, in an attempt to terminate the arrhythmia. If unsuccessful, increasing the RAP rate or increasing the duration of pacing prior to rate reduction may be helpful. Although the likelihood of inducing a destabilizing arrhythmia is remote, patients are advised to remain in bed during this procedure.Complications of temporary cardiac pacing include infection, vessel or chamber perforation, tamponade, damage to the myocardium or surgical anastomoses, as well as the potential for microshock.3,4,7 The risk of infection is reduced with meticulous aseptic technique; gauze dressings are changed per the institutional policy to keep insertion sites clean and dry, and showering is avoided while pacing leads remain in place. To avoid conducting electricity via the wires that could precipitate an arrhythmia, nonsterile gloves are worn whenever handling pacing leads, which are covered with insulating material such as a rubber glove when exposed. Surgical placement of epicardial leads avoids transvenous placement and contributes to their relative stability; therefore, no mobility restrictions are imposed for temporary pacing leads. However, providers must be mindful of generator location to avoid traction and accidental lead removal during activity. Nursing assessment specific to temporary pacing includes ensuring that all device connections are secure and battery life is adequate, pacing modes/settings are appropriate, and the ECG is reviewed carefully for problems related to sensing, pacing, or capture. Electrocardiographic monitor features that enable pacing should be activated to promote alarm triggering for loss of capture, should it occur. Monitoring focuses on detection of arrhythmias, pacing malfunction, and recovery of intrinsic rhythm, which can lead to “competition” with the device, necessitating adjustments. Pacemaker troubleshooting may require connecting wires to a generator that was disconnected, and emergent adjustments in device parameters or use of the “EMERGENCY” button. Providers are advised to limit the use of tape and particularly transparent dressings when “capping” unused wires, to facilitate the timely reconnection of pacemaker wires in such emergencies. In the event of an asystolic arrest when pacing wires are present, European cardiac surgical guidelines now recommend reinitiating pacing as the highest priority for initial resuscitation efforts. Cardiac compressions may be held for up to 1 minute to attempt emergency pacing via the epicardial wires or defibrillation of shockable rhythms.15,16 The Society of Thoracic Surgeons is currently developing a similar guideline for the United States.There is no standard duration for maintaining temporary pacemaker wires; most patients will never require pacing and those who do typically recover their underlying rhythm soon after surgery. Device weaning may be facilitated by gradually reducing the pacing rate, while carefully assessing patient tolerance and their ability to sustain a nonpaced rhythm. Removal of temporary pacemaker leads is typically accomplished with the application of gentle traction, allowing the electrode to dislodge from the heart and exit the chest wall completely. Alternatively, the lead can be pulled flush with the skin, cleansed, and clipped, retaining a portion of the wire inside the chest. A recent review recommends complete removal of leads whenever feasible, citing complications that ranged from local skin abscess, to wire migration or endocarditis, often requiring surgical removal years later.17 Retained leads are not a consideration for subsequent magnetic resonance imaging, which has been deemed safe in such patients.18 Cardiac tamponade is a rare but potentially life-threatening complication of pacemaker wire requiring in less than of Although of for several removal is et demonstrated that was present in only of patients with in patients it was or and in it the They additional of tamponade wire removal chest and A recommends the patient for after lead removal in the or when surgical is of and ensuring an less than in patients epicardial pacing leads are routinely placed during cardiac surgery, potential of cardiac output arrhythmia and and cardiac that may be They are not and when used can precipitate or cause bleeding with with these patients should of pacing and device troubleshooting and their on postoperative cardiac When atrial leads are with an atrial ECG and the use of RAP to terminate atrial are and of device malfunction, with an adequate knowledge to at the is essential to maintain patient